Day 162 Color

Nature of Color

Light and Matter

  • Differentiate between transparent, translucent, and opaque
  • Colors of visible spectrum range from red (long wavelength) to violet (high frequency)

Primary Colors of Light

  • Green, red, and blue mixed together give white light
  • Red + green = yellow
  • Green + blue = cyan
  • Red + blue = magenta
  • Yellow, cyan, magenta = secondary light colors

 Secondary Light Colors

  • Complementary colors = two light colors which form white light
  • Examples: yellow + blue; cyan + red; magenta + green
  • All of the above are examples of the additive process

 Subtractive Process

  • We see colors because some colors are reflected and some are absorbed (dyes and pigments are responsible for this)
  • Example: White light incident on a tomato: the tomato absorbs the blue and green and reflects the red

 More Examples of Colored Objects

  • Red light incident on a red rose: rose appears red because red reflects to our eyes; leaves absorb the red and become warmer than the petals because no red pigment is contained in the leaves
  • Green light on a red rose: petals look black because petals absorb the green since there is no red incident light to reflect back
  • Yellow daffodil illuminated with red light appears red
  • …illuminated with yellow appears yellow
  • …illuminated with green appears green
  • …illuminated with blue appears black

 Why is the Sky Blue?

  • Violet light is scattered the most (shortest wavelength) but our eyes are more sensitive to blue
  • When more dust particles are present, lower frequencies are scattered more
  • After a rain, sky appears more blue due to absence of dust
  • At greater altitudes there are less particles and eventually where there are no particles, the “sky” is black
  • Clouds are made of different sized clusters of water molecules, ranging from low to high frequencies – overall effect is a bright white cloud

 Why are Sunsets Red?

  • Red light is scattered the least, has the longest wavelength
  • Lowest path from sun to earth is at sunrise and sunset, hence more red reaches at these times, rather than higher frequencies
  • If dust is present, longer red wavelengths diffract producing even more red

 Why is Water Blue-Green?

  • Water molecules absorb infrared waves because they resonate to those frequencies
  • Water molecules resonate weakly to visible red frequencies causing a gradual red absorption by water
  • When red is removed from the white sunlight, greenish-blue remains